An instantaneous method for measuring leakage flow rate was proposed by employing a standard flow. Measuring equipment was connected to the pipeline, which promised the convenience and non-destruction. Standard flow was used to determine the internal volume of measured equipment. And a nonlinear discrete-time tracking differentiator had been designed to track the differential pressure. Furthermore, an algorithm was formulated to describe, the relationship between leakage and standard flow rate. The method deviates from the theoretical leakage values by less than 3% full scale (FS), and shows a good precision and scope compared with traditional flow measurements. In addition, the proposed parallel connection based on standard flow makes operation easy and measuring fast possibly, thus the method can be applied in the new area of pneumatic energy saving industry.